The most basic relationship is that they are sisters.
Dee is the older sister, Maggie the younger.
However, there is more to them than this. Dee is the star: the family member who went away. She left the family’s modest rural home, and embraced the waves of change that were moving through America. She represents a specific current of African American identity; she chooses an African heritage and a new name: “Wangero Leewanika Kemanjo.” She is proud of who she is, and of the conscious choices she’s made.
Maggie, by contrast, stayed at home. She’s quiet and shy, and scarred from the fire. She doesn’t have the same sort of pride that Dee has, but she is loyal to her lived and experienced heritage, something their mother endorses by giving her the quilt.